Why did I even go there in my mind?

So I've been lucky enough this semester to end up having 2 ED (Emotionally Disturbed) students. In esence this means that they are kids with major behavioral problems. The one I've had all semester hasn't caused any problems in the class and I don't think he would ever do anything drastic to put the class in danger, but this new kid makes me nervous. He's been in my class for about a week and has come to class maybe 3 times. He sits in the back of the class with his head down and a big black hoody pulled over his head (I think you know were I'm going). He's eriely quiet and for some reason the horrible thought of him doing something crazy popped in my head. Obviously there's a slimmer than slim chance of him doing the unthinkable, but for some reason I couldn't get it out of my head. What would I do!? The class is not even in the main campus. It would take at least a few minutes to get security. The thought of him actually hitting someone and maybe then doing it to himself. I know I say my kids are brats, but the thought of them getting hurt would kill me inside. I would not know what to do. I'd be frozen. That Pearl Jam song "Jeremy" plays on repeat whenever I hear about a school shooting. All I can say to new secondary school teachers is, don't go there. don't think about it. and pray we will NEVER have to go through it...